The present invention relates to a resonator and in particular to a novel resonator structure having a symmetric frequency response with one major resonant frequency response and low spurious frequency response with no etched grooves or recessed electrodes, with a minimum electrode geometry of .lambda./4 with single level fabrication and uniform wave velocity throughout the structure and frequency insensitivity to electrode reflections. In addition, the present invention also relates to a novel resonator filter utilizing the construction principles of the novel resonator.
Current resonator designs use synchronously placed transducers with two electrodes per wavelength to achieve insensitivity to electrode reflections. The aperiodic space between the two transducers forms a resonant cavity. The design of the structure is one of physical symmetry having first and second reflective gratings separated by spaced first and second transducers with the gratings and the transducers being identical. The transducers are typically overlap weighted to realize a cosine like transverse weighting to suppress transverse modes. All electrodes and reflectors are typically single level aluminum. Such a structure has a uniform velocity with a minimum geometry of the electrodes, and the gaps between the electrodes, being quarter wavelength. Further, no grooves or recessed electrodes are required and the synchronous placement of the transducers yields a resonator whose frequency is independent of the reflection coefficient of the electrodes per unit length of the transducers. Such a structure has the disadvantages that the transducers are not positioned for maximum coupling to the standing waves generated in the substrate and the frequency response is asymmetric with high out-of-band spurious responses. There is an especially high peak on the low frequency side of resonance which can be a serious problem. For instance, if the beam width of the resonator is increased, the insertion loss at resonance can be decreased somewhat. However, the low frequency side spurious response increases rapidly thus resulting in a lower value in the out-of-band rejection capability of the resonator. Further, the level of the low side spurious response can increase in a different impedance system than the design impedance. If the loss at the low frequency side due to the spurious response decreases significantly, the oscillator in which such resonator may be used may jump to this frequency instead of the resonant frequency. It would thus be advantageous to have a symmetric frequency response with high out-of-band rejection. This would eliminate the frequency hop problem. Further, such a resonator could be used as an improved element of a coupled resonator filter.
Such a desired novel resonator structure providing the desired symmetric frequency response would require that the structure have no etched grooves or recessed electrodes, the minimum electrode geometry be .lambda./4 (no split finger electrodes), the structure have single level fabrication, the velocity of the wave form be uniform throughout the structure and the frequency be insensitive to electrode reflections.
The present invention overcomes the problems of the prior art resonators by utilizing a structure which has no etched grooves or recessed electrodes, has minimum electrode geometry of .lambda./4, has single level fabrication, has uniform wave form velocity throughout the structure and is insensitive to electrode reflections. This is accomplished on a standard or conventional crystal cut orientation by removing the resonant cavity that normally exists between the two transducers and placing that resonant cavity between one of the transducers and its associated grating. As the frequency decreases, the resonant cavity associated with the nearest transducer causes a shift in the standing waves under the nearest transducer electrode towards the space between the electrodes thus decreasing the coupling between the electrodes and the standing waves and causing a null. In like manner, as the frequency increases, the other transducer and its associated grating also shifts its standing waves toward the spaces between the electrodes thus causing a null on the high side of the frequency response. Such is the operation of the novel transducer when it is placed on a normal or standard crystal cut.
If the novel structure is placed on a natural single phase unidirectional transducer (NSPUDT) cut, that is, a crystal cut as explained in copending application Ser. No. 677,513 entitled Single Phase Unidirectional Surface Acoustic Wave Transducer and filed Dec. 3, 1984 and incorporated herein by reference and in which it was disclosed how to determine a crystal orientation such that there is a .lambda./8 or 45.degree. separation between the centers of transduction and reflection, then at resonant frequency the standing waves are maximal under each electrode of both transducers and thus the output of the resonator is a maximum. However, as the frequency shifts either to the high or the low side, the standing waves move away from the centers of the electrodes toward the spaces on either side therefore causing a null to appear whether the frequency is increased or decreased. The output of this resonator structure is therefore symmetrical. However, because one of the transducers is close to the resonant cavity and the other one is further removed from the resonant cavity, there is a difference in coupling to the cavity. Therefore, if the number of electrodes in the near transducer are reduced and the number of electrodes in the far transducer are increased, the resonator output can be maximized.
In addition, by using the novel resonator structure in a resonator filter, an improved resonator filter can be obtained by placing the novel structure on a crystal having a conventional or standard orientation crystal cut and can be further improved by placing the novel resonator structure in a resonator filter formed on an NSPUDT crystal cut to obtain an improved resonator filter.
Thus, it is an object of the present invention to provide a novel resonator structure which has a symmetrical frequency response and one major resonant frequency response and low spurious frequency responses and which has no etched grooves or recessed electrodes, a minimum electrode geometry of .lambda./4 (without split finger electrodes) single level fabrication and a uniform wave velocity throughout the structure and frequency insensitivity to electrode reflections.
It is a further object of the present invention to provide a novel resonator in which the resonant cavity is formed between one of said transducers and its associated reflective grating structure.
It is still another object of the present invention to provide a novel resonator structure in which the resonant cavity is formed between one of said transducers and its associated reflective grating structure and the entire structure is formed on a crystal cut of normal crystal orientation.
It is yet another object of the present invention to provide a novel resonator in which the resonant cavity is formed between the input transducer and its associated reflective grating structure and the entire transducer structure is formed on a natural SPUDT crystal cut.
It is another object of the present invention to minimize the insertion loss of the novel resonator by decreasing the number of electrodes in the transducer nearest the resonant cavity and increasing the number of electrodes in the transducer furtherest from the resonant cavity thereby balancing the coupling between the transducers and the resonant cavity and maximizing the output of the resonator.
It is also an object of the present invention to utilize the novel resonator structure to form a resonator filter on both a normal crystal cut and on a natural SPUDT crystal cut.